Aircraft engines generate a substantial amount of noise. In high population areas and other noise controlled environments, aircraft engine noise can be problematic. In order to reduce the noise generated by modern aircraft engines, aircraft manufacturers often line the aircraft engine nacelle and surrounding engine areas with acoustic liners that at least partially suppress the amount of noise radiated by the engines to the surrounding environment. Such acoustic liners typically include honeycomb composite sandwich panels configured to absorb or dissipate sound energy. As used herein, the term “honeycomb” refers to an array of open cells formed by a matrix of interconnected cell walls. Though such cells often have hexagonal cross-sections (like that of a honeycomb built by honeybees), composite acoustic sandwich panels also can include arrays of cells with other polygonal or non-polygonal shapes. Accordingly, the term “honeycomb” is used herein to refer to arrays of open cells that having no particular shape or cross-section. Often, the cell walls of acoustic honeycomb cores are constructed of thin metal foil such as aluminum or titanium.
An acoustic liner panel typically includes an imperforate backing sheet covering one face of the honeycomb core, and an air permeable face sheet covering the opposite face of the core. The air permeable face sheet permits sound energy to propagate through the face sheet and into the open cells, where at least some of the sound energy is dissipated and/or absorbed. Often, the air permeable face sheet is a thin sheet of material having a plurality of spaced perforations or openings therethrough. In order to maximize the sound absorption capability of an acoustic liner, it is desirable to maximize the number of open cells in the liner that are capable of receiving sound waves through the air permeable face sheet. Accordingly, it is desirable to minimize the number of cells and openings in the face sheet that are wholly or partially obstructed. In other words, it is desirable to maximize the acoustically treated area of an acoustic liner in order to maximize the noise suppression capability of the liner.
An acoustic liner typically includes a plurality of acoustic segments or panels. Often, adjoined edges of such segments or panels are connected together by a foaming adhesive material. Such foaming adhesives can be used to join new acoustic panel sections, or to attach replacement honeycomb sections or patches during repair of damaged acoustic panels. As shown in FIG. 1, the foaming adhesive 10 typically at least partially fills cells 12 along adjoined edges 14, 16 of the acoustic cellular core sections 18, 20. The foaming adhesive 10 also can at least partially fill other cells 26 adjacent to the adjoined edges 14, 16. The foaming adhesive 10 also typically blocks at least some openings 22 in the perforated inner skin 24 along the joint. Accordingly, the foaming adhesive 10 effectively prevents sound waves from entering the blocked and partially blocked cells 12, 26 of the core sections 18, 20. As a result, the sound suppression capability of the associated acoustic liner is less than it would be if the openings 22 and cells 12, 26 were unblocked by the foaming adhesive 10.
Accordingly, there is a need for an alternative method of joining abutting edges of acoustic honeycomb sandwich panel sections that has little or no impact on the sound suppression properties of the panel sections. Preferably, such a method will provide a joint construction that obstructs substantially none of the affected cells or face sheet openings along the joint, thereby maximizing the total acoustically treated area of a multi-section acoustic liner.